1. Field of the Invention
Embodiments disclosed here generally relate to obtaining a sample. More particularly, the present disclosure relates to apparatus and methods to obtain a sample of sediment from the bottom of a produced fluid storage container.
2. Background Art
Many oilfield installations located off-shore, particularly in the North Sea, use produced fluid storage containers made of concrete that are located on the sea bed. The storage containers are designed to contain fluids produced from wells drilled deep into a reservoir and to allow separation of crude oil from co-produced connate water. Typically, there are many large storage containers for each offshore installation; each container may be around ten meters in diameter and over fifty meters deep.
The produced fluid is pumped into the storage container through an import/export pipe located near the top of the storage container. The produced oil has a lower density than the produced connate water and, thus, separates and forms a layer on top of the water within the storage container. Once separated, the oil in the storage containers may be exported from the storage container via the import/export pipe located near the top of the storage container. There is a dead volume between the export pipe and the top of the storage container that may be occupied by crude oil and gasses. As the oil is removed from the produced fluid storage container, its volume is replaced by seawater which enters via a pipe located near the bottom of the storage container. The seawater may be stored in a header tank which maintains a hydrostatic pressure of several bars depending on the design of the storage containers.
When the production of oil from the reservoir is no longer viable, there is a need to decommission the platform and clean up the storage containers. To remove all oil from the storage container, the process of displacing oil with seawater continues until only the dead volume of hydrocarbon gasses and seawater remain. To remove the hydrocarbon gasses, a gas lighter than the hydrocarbon gas, preferably carbon dioxide, is introduced into the storage container to displace the hydrocarbon gas. Only the displacement gas and seawater remain in the storage container after the hydrocarbon gas is removed through the export pipe. Finally, the contents of the storage container are treated with a chemical solution to absorb the displacement gas and to convert the remaining contents of the storage container into briny water. However, in most cases, the storage container further contains sediment which prevents safe disposal of the container.
Sediment build-up occurs over many years of production operation. A variety of solid wastes can be introduced into or form within the storage containers resulting in the development of a layer of sediment. For example, the mixing of incompatible brines, such as produced connate water and seawater, can result in the precipitation of mineral scales, such as barium sulphate or calcium carbonate to the bottom of the storage container, and naphthenic acids present in the crude oil can react with calcium-rich brine to form calcium naphthenate deposits. Sand and mud may also be produced which settle to the bottom of the storage container. The accumulated sediment may require special handling and disposal methods due to hydrocarbon materials and naturally occurring radioactive minerals that may be entrained therein. Naturally occurring radioactive minerals such as radium-226 sulphate, lead-210 metal, and a series of other radioisotopes resulting from the decay of uranium and thorium may be present in the water contained within the pores of reservoir rock. A sample must be obtained to determine the composition of the sediment in order to determine the proper method for removal or treatment of the storage container.
The only access to the inside of the storage container is typically through the import and export pipes that have an inner diameter typically around 10-12 inches and follow convoluted paths often containing several 90 degree bends. Furthermore, the storage containers lie in deep water such that the top of the containers may be more than 100 meters below the surface of the sea, thus rendering problematic the option of drilling into the container to retrieve a sample.
Accordingly, there exists a need for a sampling device capable of passing through sharp bends in the import and export pipes leading to and from a produced fluid storage container located on a sea floor. Furthermore, there exists a need for a sampling device designed to locate sediment, measure the depth of the sediment, collect a sample, and return to a desired altitude within the container for retrieval without operator input.